Add insallation instructions.

Author: judgej

README.md

@@ -10,6 +10,7 @@ Table of Contents
 
    * [Table of Contents](#table-of-contents)
    * [Omnipay-AuthorizeNetApi](#omnipay-authorizenetapi)
+   * [Installation](#installation)
    * [Authorize.Net API](#authorizenet-api)
       * [API Authorize/Purchase (Credit Card)](#api-authorizepurchase-credit-card)
       * [API Capture](#api-capture)
@@ -24,6 +25,10 @@ Table of Contents
 
 Omnipay 3.x implementation of Authorize.Net API
 
+# Installation
+
+    composer require "academe/omnipay-authorizenetapi: ~3.0"
+
 # Authorize.Net API
 
 The *Authorize.Net API* driver handles server-to-server requests.
@@ -217,13 +222,19 @@ the previous section. In the `creditCard` object, leave the card details
 blank, not set. Instead, send the opaque data:
 
 ```php
-$gateway->authorize([
+$request = $gateway->authorize([
     ...
     'opaqueDataDescriptor' => $opaqueDataDescriptor,
     'opaqueDataValue' => $opaqueDataValue,
 ]);
 ```
 
+or
+
+```php
+$request->setOpaqueData($opaqueDataDescriptor, $opaqueDataValue);
+```
+
 The authorizatiob or purchase should then go ahead as though the card
 details were provided directly. In the result, the last four digits
 of the card will be made available in case a refund needs to be performed.